As DRAMs become more highly integrated, the spacing between individual memory cells decreases. This causes many design and engineering challenges, not the least of which is in photolithography. In a photolithography process, a resist is coated on a wafer and then exposed to light via a reticle having a pattern formed on a surface of the photomask. The resist is then developed. Thus, a pattern corresponding to the pattern on the reticle is formed in the resist.
An integral component of a photolithographic apparatus is the reticle which includes the pattern corresponding to features at one layer in an IC design. The reticle typically includes a transparent quartz plate covered with a patterned light-blocking material such as chromium. The reticle is placed between a radiation source producing radiation of a pre-selected wavelength and a focusing lens which may form part of a "stepper" apparatus. Placed beneath the stepper is a resist covered silicon wafer. When the radiation from the radiation source is directed onto the reticle, light passes through the quartz (regions not having chromium patterns) and projects onto the resist covered silicon wafer. In this manner, an image of the reticle is transferred to the resist.
However, as the wavelength of light becomes large relative to the dimensions of the feature sizes on the reticle, the light passing through the reticle becomes refracted and scattered by the chromium edges. This causes the projected image to exhibit some rounding or other optical distortion. While such effects pose relatively little difficulty in layouts with large feature sizes (for example, layouts with critical dimensions above about 1 micron), they cannot be ignored in layouts having features smaller than about 1 micron. The problems become especially pronounced in IC designs having feature sizes near the wavelength of light used in the photolithographic process. The distortion caused by the refraction, diffraction, and scattering is referred to generally as the optical proximity effect (OPE). Unfortunately, any distorted illumination pattern propagates to a developed resist pattern and ultimately to IC features such as the bit line and other interconnects. As a result, the IC performance is degraded or the IC becomes unusable.
To remedy this problem, a reticle correction technique known as optical proximity correction (OPC) has been developed. OPC involves adding dark regions from a reticle design at locations chosen to overcome the distorting effects of diffraction and scattering. Another known technique to diminish the optical proximity effect is to employ a phase-shifting mask. Another technique is to employ deformation illumination such as oblique illumination.
Although these known techniques can reduce the optical proximity effect thereby improving the pattern transfer characteristics, in practice however, in the phase-shifting mask technique in which the phase of the exposure light is controlled and also in the deformation illumination technique in which the order of diffraction of the exposure light is controlled, these techniques are difficult to implement or still do not provide the necessary correction to the optical proximity effect.
In the DRAM manufacturing context, one of the necessary steps in manufacturing a DRAM is the masking and etching of repetitively patterned interconnects, such as the bit line. FIG. 1 shows a conventional reticle used to etch the bit line for a DRAM. As seen, via pads 101 are periodically placed in parallel interconnect lines 103. As the spacing between the parallel bit lines decreases, the optical proximity effect will tend to distort the bit line pattern.
What is needed is a new method for correcting for the optical proximity effect, particularly in a bit line mask used in the manufacture of DRAM devices.